The invention relates to a process of gauging and control of adaptation of impedance for low noise reception chains for a device for measuring microwave noise. The invention will be applied particularly in micro-wave thermography carried out in a monolithic integrated circuit for measuring the internal temperature of bodies.
The principle of microwave thermography for measuring temperature in depth of the order of some centimeters is well known. It is an item of equipment which comprises in particular an aerial which is intended to pick up the thermal radiation given out by a certain volume of the body to be examined.
It is known that there is a direct relationship between the microwave signal and the internal temperature of the body.
Several devices of this type have been developed, such as the Dicke radiometer in order to enable the thermal radiation to be analysed.
In general, the principle which is used may be summarised as follows: The hyperfrequency signal picked up by the aerial resulting from the thermal radiation of the object to be analysed which is in proportion to the temperature of the object is compared alternatively with a signal coming from a standard noise source or temperature resistance which is constantly checked and measured.
For this purpose, a switch controlled by an alternating LF (low frequency) signal makes it possible to "connect" the signal coming from the noise source or resistance or that coming from the hyperfrequency aerial alternatively at the input of a hyperfrequency amplifier.
The alternating signal which is received and detected at the output from the amplifier is thus in proportion to the temperature difference between that of the object to be measured and the source of noise. This signal may, therefore, constitute an error signal enabling the setting of the temperature of the standard noise source or resistance to be adjusted so as to achieve equality of the two temperatures. All that is then necessary is to read the adjusted temperature of the standard noise source or resistance in order to obtain that of the object to be measured.
It should, however, be noted that, in order to obtain an exact measurement of the temperature of the object, it is necessary to know exactly the impedance presented by the aerial which picks up the thermal radiation and to adjust strictly that of the noise source or resistance, the temperature of which should be adjusted and checked strictly.
In fact, the noise temperature of the amplifier depends on the impedance of the input device and it is, therefore, essential for the resistance of the aerial to be equal to that of the noise source or resistance.
At the present time, this measuring is extremely delicate and calls for a particularly delicate prior manipulation.
The purpose of the present invention is to present a process of impedance adaptation control for a low noise reception chain such as the radiometers used in microwave thermography which makes it possible, at all times, to ensure exact correspondence between the impedance presented by the aerial and the impedance presented by the standard temperature reference or source of noise.